Dabbing Out Blue Marking Pens
If your blue marking lines have turned brown, to remove use a gallon of plain white vinegar in the machine with the wet quilt. (If the laundry detergent has sodium carbonate in it, that
is the 'fixer' for the color in the dye. This sodium carbonate, otherwise known as soda ash, can be found in its purest form in Arm & Hammer or any other detergent with 'whiteners and brighteners.' The reason that vinegar helps remove the stain is because it pushes the pH of the water to an acidic point, thus allowing the fabric to release the dye. (Sodium carbonate is a strong base.)
Remember to soak/rinse your quilt in plain cold water before you wash it! That way, the pen marks will come out. After the quilt is thoroughly rinsed and then washed, the pen marks will never come back at a later date, or after drying.
Some quilters keep a spray bottle of plain water beside them and as they finish a block, spritz it right away. That way they won't forget and it is ready to launder. Some keep a damp paper towel nearby and as soon as they finish a section, dab at the blue marks until they disappear.